Apparatus and methods of operating a combustion engine

ABSTRACT

The present invention generally relates to dual chamber combustion engines. The invention particularly relates to apparatus and methods for ignition of ultra-lean fuel/air mixtures with fuel/air mixture streams traveling at supersonic velocities. Present invention will be impactful in emission reduction as well as improving fuel economy and thermal efficiency in engines.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present U.S. patent application is related to and claims thepriority benefit of U.S. Provisional Patent Application Ser. No.62/414,854, filed Oct. 31, 2016, the contents of which are herebyincorporated by reference in their entirety into the present disclosure.

TECHNICAL FIELD

The present invention generally relates to dual chamber combustionengines. The invention particularly relates to apparatus and methods forignition of ultra-lean fuel/air mixtures with fuel/air mixture streamstraveling at supersonic velocities.

BACKGROUND

This section introduces aspects that may help facilitate a betterunderstanding of the disclosure. Accordingly, these statements are to beread in this light and are not to be understood as admissions about whatis or is not prior art.

Combustion of lean fuel mixtures, that is, mixtures having a relativelylow fuel-to-air mixing ratio, may provide for reductions in emissionsfrom internal combustion engines of the types used in the engine andpower generation industries. Dual chamber engines, for example, of typesthat may be used in turbine engines, utilize a pre-chamber and a mainchamber. In general, a small quantity of stoichiometric fuel/air isinitially burned in the pre-chamber. The resulting combustion productsare then discharged in the form of a hot turbulent fuel/air mixturestream (jet) through a small diameter nozzle into the main chamber,which is filled with leaner fuel/air mixture. Compared to a conventionalspark plug, the turbulent jet has a much larger effective “surface area”leading to multiple ignition sites on its surface that can enhance theprobability of successful ignition. The turbulence brought by the hotjet can cause faster flame propagation and heat release. A non-limitingexample of a type of a dual chamber power turbine engine is described inU.S. Pat. No. 4,292,801 to Wilkes et al.

Decreasing the fuel-to-air mixing ratio of a fuel mixture used inengines can reduce emissions, for example, NOx, unburned hydrocarbon(UHC), particulate matters, and improve thermal efficiency. However,ignition of ultra-lean fuel/air mixtures can be challenging and maycause engine misfires. As used herein, ultra-lean fuel/air mixtures aredefined as having a fuel/air equivalence ratio that is lower than thetypical values used in current combustion engines, wherein theequivalence ratio is the ratio of the fuel-to-oxidizer ratio to thestoichiometric fuel-to-oxidizer ratio.

Despite advances in lean burn combustion, there is an ongoing desire toreduce emissions from engines. In particular, there is a desire for anengine capable of reliably combusting ultra-lean fuel mixtures.

SUMMARY

The present invention provides apparatus and methods suitable forreliably combusting ultra-lean fuel mixtures in combustion engines.

In one embodiment, the present disclosure provides a supersonic ignitiondevice comprising a supersonic nozzle, wherein the supersonic nozzle hasa converging-diverging geometry and an area ratio between four and nine,and the length of the supersonic nozzle is 10-30 mm, and the supersonicnozzle is configured to be capable of igniting an ultra-lean fuel-airmixture.

In one embodiment, the present disclosure provides a supersonic jetcombustion engine comprising a pre-chamber, a supersonic nozzle with alength between 10-30 mm, and a main chamber, wherein the supersonicnozzle has a converging-diverging geometry and an area ratio (defined asthe value of the exit area of the nozzle divided by the throat area ofthe nozzle) between four and nine, wherein the supersonic jet combustionengine is configured to generate a jet of combustion products from afirst fuel/air mixture in the pre-chamber, through the supersonicnozzle, and into the main chamber with at least supersonic velocity (atleast Mach 1) to ignite a second fuel/air mixture in the main chamber,wherein the second fuel/air mixture is an ultra-lean fuel mixture with afuel/air equivalence ratio of equal to or less than 0.4.

In one embodiment, the present disclosure provides a method of operatinga supersonic jet combustion engine, wherein the method comprises: a)igniting a first fuel/air mixture in a pre-chamber; b) introducing a jetof combustion products from the first fuel/air mixture from thepre-chamber, through a supersonic nozzle with a length between 10-30 mm,and into a main chamber with at least supersonic velocity (at least MACH1), wherein the supersonic nozzle has a converging-diverging geometryand an area ratio between four and nine; wherein the jet with supersonicvelocity causes combustion of a second fuel/air mixture in the mainchamber, wherein the second fuel/air mixture is an ultra-lean fuelmixture with a fuel/air equivalence ratio of equal to or less than 0.4.

Technical aspects of the method described above preferably include thecapability of reliably combusting ultra-lean fuel mixtures, therebyimproving fuel economy and combustion efficiency and potentiallyreducing NOx emissions in combustion engines.

Other aspects and advantages of this invention will be furtherappreciated from the following detailed description.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thepresent disclosure, reference will now be made to the embodimentsillustrated in the drawings, and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of this disclosure is thereby intended.

In the present disclosure the term “about” can allow for a degree ofvariability in a value or range, for example, within 10%, within 5%, orwithin 1% of a stated value or of a stated limit of a range.

In the present disclosure the term “substantially” can allow for adegree of variability in a value or range, for example, within 90%,within 95%, or within 99% of a stated value or of a stated limit of arange.

In present disclosure the term “ultra-lean fuel mixtures” may refer to afuel mixtures that has a fuel/air equivalence ratio that is lower thanthe typical values used in current combustion engines. In some aspect,the ratio is below 0.40, 0.35, 0.30, 0.29, 0.28, 0.27, 0.26, 0.25, 0.24,0.23, or 0.22. In some aspect, the ratio may even approach the leanflammability limit of such fuel in air.

In the present disclosure, the term “area ratio” may refer to the valueof the exit area (A_(e)) of the nozzle divided by the throat area(A_(t)) of the nozzle. For the purpose of the present disclosure, thearea ratio is required to be in the range of 4-9. The throat is thejoint section of the converging part and the diverging part of aconverging-diverging supersonic nozzle.

If not specifically defined, other terms such as pre-chamber, mainchamber, and other terms should be interpreted as meanings generallyaccepted by a person with ordinary skill in the art.

Conventional combustion engines that utilize a dual chamber system, suchas but not limited to power turbines of the type, generally introduce afuel/air mixture stream (jet) through a fuel nozzle from the pre-chamberinto the main chamber at subsonic speeds, that is, at a rate of travelthat is below the speed of sound. In addition, the traditionalconvergent-divergent nozzle (also referred as DeLaval nozzle) used inspace application normally has an area ratio of about 20-160 due to needto use a vigorously high-speed (typically a few thousand meters persecond) jet to produce thrust. Disclosed herein are apparatus andmethods of operating dual chamber engines which include a fuelconvergent-divergent supersonic nozzle that introduces the jet from thepre-chamber into the main chamber at supersonic speeds, that is, at arate of travel that is at or above the speed of sound (Mach 1).Surprisingly, the area ratio for the convergent-divergent supersonicnozzle is required to be in the range of 4-9 to generate a supersonicjet capable of igniting ultra-lean fuel mixtures within the mainchamber. Such engines are then capable of combustion of ultra-lean fuelmixtures within the main chamber. Although the engines and fuel nozzlesdisclosed herein will be described in reference to the combustion ofhydrogen/air fuel mixtures, it should be understood that the teachingsdisclosed herein are applicable to various other fuel/air mixtures, suchas but not limited to natural gas/air fuel mixtures, and are thereforealso within the scope of the invention.

According to one aspect of the invention, by introducing a jet into amain chamber at supersonic speeds, the lean flammability limit ofhydrogen/air in comparison to subsonic jets may be extended, that is, anengine is capable of combusting fuel mixtures having relatively lowerfuel-to-air mixing ratios. For example, under conditions present duringinvestigations leading to the present invention, a hydrogen/air leanlimit of a main chamber mixture achieved by introducing jets at subsonicspeeds resulted in an equivalence ratio of 0.31. In comparison,introducing jets at supersonic speeds extended (reduced) this limit toan equivalence ratio of 0.22 without any significant increase inignition delay time. These results indicated that the main chambermixture can be leaner (that is, have a relatively lower fuel-to-airmixing ratio) with the use of supersonic jets. Therefore, hydrogen/airengines that introduce jets into their main chamber at supersonic speedscan achieve a hydrogen/air lean limit of their main chamber mixture ofequal to or less than 0.4, and preferably equal to or less than 0.22.

The design of converging-diverging (C-D) nozzle plays a crucial role togenerate hot supersonic jet that could ignite a ultra-lean hydrogen/airmixture. Since the dimension of the nozzle is very small, the heattransfer through the nozzle wall becomes severe. If the heat transfer istoo much, the enough heat may get lost through the nozzle wall and wouldfail to ignite the lean mixture in the main chamber. Therefore, thetotal length of the C-D nozzle should not be more than 25 mm. Foroptimized operation, the C-D nozzle should not be more than 15 mm.However, the length of the nozzle cannot be shorter than 10 mm since togenerate a fully developed supersonic jet without any boundary layerseparation, 10 mm is the least required length. Nozzle entrance andthroat have been smoothened by providing chamfer. Further, the diameterof the throat is also critical. One requirement for the presentdisclosure is to generate a jet that can sustain for a reasonably longtime. Since pre-chamber has a constant amount of mass after combustion,the flow rate depends on the nozzle diameter. Bigger diameter meanslarger flow. This means the jet lifetime would be short. Also the jetvelocity will be small. In short, the jet will be ineffective. A smallerdiameter also means area to volume ratio will be too large. The jet willlose too much heat to make the ignition ineffective. The diameter of thethroat should be within a range of about 0.5 mm-3.0 mm. A preferredrange is about 1.0 mm-2.0 mm. A more preferred range is about 1.25mm-1.75 mm.

In general, the introduction of subsonic jets into the main chamberresults in a monotonic decrease in the static temperature in adownstream location. In contrast, supersonic jets cause shock structuresthat result in the static temperature fluctuating and then increasing atthe downstream location. The increase in the static temperature behindthe shock structures escalates ignition probability, which leads to areduction in the lean flammability limit.

This reduction in the lean flammability limit was counterintuitive.Since supersonic jets have relatively higher velocities, it wasconsidered possible that they could result in rapid mixing at a ratethat is too high for ignition to occur. However, investigations leadingto the invention determined that ultra-lean supersonic jets can resultin fast and reliable ignition.

Various nozzle geometries were evaluated during the investigationsleading to the present invention. Nozzles having converging-diverging(C-D) geometries were observed to outperform nozzles having straight orconverging geometries. Intuitively, with regard to C-D nozzles, sincetemperature rise increases with an increase in the area ratio of anozzle, it was expected that performance would improve with an increasein the area ratio of the nozzles. However, the investigations indicatedthat the highest tested area ratio nozzles did not substantially extendthe lean flammability limit. Rather, a range of nozzle area ratiosbetween about four and nine resulted in an extension of the leanflammability limit, that is, the lowest equivalence ratios, and showedimproved performance relative to higher area ratio nozzles.

In one embodiment, the present disclosure provides a supersonic ignitiondevice comprising a supersonic nozzle, wherein the supersonic nozzle hasa converging-diverging geometry and an area ratio between four and nine,and the length of the supersonic nozzle is at least 10 mm.

In one embodiment, the present disclosure provides a supersonic ignitiondevice comprising a supersonic nozzle, wherein the supersonic nozzle hasa converging-diverging geometry and an area ratio between four and nine,and the length of the supersonic nozzle is 10 mm-30 mm. In one aspect,the length is 10 mm-20 mm, or 10 mm-15 mm.

In one aspect of the supersonic ignition device, the length of thediverging section is longer than converging section.

In one aspect of the supersonic ignition device, the diameter of thethroat is about 0.5 mm-3.0 mm. In one aspect, the diameter of the throatis about 1.0 mm-2.0 mm. In one aspect, the diameter of the throat isabout 1.25 mm-1.75 mm.

In one embodiment, the present disclosure provides a method of ignitinga fuel-air mixture by a supersonic ignition device comprising asupersonic nozzle, wherein the supersonic nozzle has aconverging-diverging geometry and an area ratio between four and nine,and the length of the supersonic nozzle is at least 10 mm, wherein thevelocity of the jet released from the supersonic nozzle is at leastsupersonic speed (Mach 1). In one aspect, the length of the supersonicnozzle is 10 mm-30 mm, 10 mm-20 mm, or 10 mm-15 mm. In one aspect, thelength of the diverging section is longer than converging section. Inone aspect, the fuel-air mixture is an ultra-lean mixture with fuel-airratio of equal to or less than 0.4. In one aspect, fuel-air mixture isan ultra-lean mixture with fuel-air ratio is 0.22-0.29. In one aspect,fuel-air mixture is an ultra-lean mixture with fuel-air ratio is0.22-0.23.

In one embodiment, the present disclosure provides a supersonic jetcombustion engine comprising a pre-chamber, a supersonic nozzle with alength between 10-30 mm, and a main chamber, wherein the supersonicnozzle has a converging-diverging geometry and an area ratio rangebetween four and nine, wherein the supersonic jet combustion engine isconfigured to generate a jet of combustion products from a firstfuel/air mixture in the pre-chamber, through the supersonic nozzle, andinto the main chamber with at least supersonic velocity (at leastMach 1) to ignite a second fuel/air mixture in the main chamber. In oneaspect, the second fuel/air mixture is an ultra-lean fuel mixture with afuel/air equivalence ratio of equal to or less than 0.4, 0.35, 0.30,0.29, 0.28, 0.27, 0.26, 0.25, 0.24, 0.23, or 0.22. In one aspect,fuel-air mixture is an ultra-lean mixture with fuel-air ratio is0.22-0.40, 0.22-0.35, 0.22-0.30, 0.22-0.29, 0.22-0.28, 0.22-0.27,0.22-0.26, 0.22-0.25, 0.22-0.24, or 0.22-0.23. In one aspect, the lengthof the supersonic nozzle is 10 mm-30 mm, 10 mm-20 mm, or 10 mm-15 mm. Inone aspect, the diameter of the throat is about 0.5 mm-3.0 mm, 1.0mm-2.0 mm, or 1.25 mm-1.75 mm.

In one embodiment, the present disclosure provides a method of operatinga supersonic jet combustion engine, wherein the method comprises: a)igniting a first fuel/air mixture in a pre-chamber; b) introducing a jetof combustion products from the first fuel/air mixture from thepre-chamber, through a supersonic nozzle with a length between 10-30 mm,and into a main chamber with at least supersonic velocity (at least Mach1), wherein the supersonic nozzle has a converging-diverging geometryand an area ratio between four and nine; wherein the jet with supersonicvelocity causes combustion of a second fuel/air mixture in the mainchamber, wherein the second fuel/air mixture is an ultra-lean fuelmixture with a fuel/air equivalence ratio of equal to or less than 0.4.In one aspect, the second fuel/air mixture has a fuel/air equivalenceratio of equal to or less than 0.35, 0.30, 0.29, 0.28, 0.27, 0.26, 0.25,0.24, 0.23, or 0.22. In one aspect, the second fuel/air mixture has afuel/air equivalence ratio of 0.22-0.40, 0.22-0.35, 0.22-0.30,0.22-0.29, 0.22-0.28, 0.22-0.27, 0.22-0.26, 0.22-0.25, 0.22-0.24, or0.22-0.23. In one aspect, the length of the supersonic nozzle is 10mm-30 mm, 10 mm-20 mm, or 10 mm-15 mm. In one aspect, the length of thesupersonic nozzle is 10 mm-30 mm, 10 mm-20 mm, or 10 mm-15 mm. In oneaspect, the diameter of the throat is about 0.5 mm-3.0 mm, 1.0 mm-2.0mm, or 1.25 mm-1.75 mm.

Experimental Setup

A small volume, 100 cc cylindrical stainless steel (SS316) pre-chamberwas attached to the rectangular (10″×6″×6″) carbon steel (C-1144) mainchamber. The main chamber to pre-chamber volume ratio was 100:1.

A stainless steel orifice plate with various nozzle designs separatedboth chambers. Jet ignition characteristics of hydrogen/air for sixdifferent nozzle designs (straight, convergent and convergent-divergent(C-D)) were studied. Nozzle dimensions are tabulated in Table 1.

A thin, 0.001″ thick aluminum diaphragm isolated both chambers withdissimilar equivalence ratios from mixing. A provision was made to heatup the fuel/air mixture in both chambers up to 600 K using built-inheating cartridges (Thermal Devices, FR-E4A30TD) inserted into the mainchamber side and bottom walls. For the present disclosure, all testswere done at room temperature 300 K. The mixture in the pre-chamber wasignited by an electric spark created by a 0-40 kV capacitor dischargeignition (CDI) system. An industrial grade double Iridium Bosch sparkplug was attached at the top of the pre-chamber. The transient pressurehistories of both chambers were recorded using high resolution ˜5 kHzKulite (XTEL-190) pressure transducers combined with NI-9237 signalconditioning and pressure acquisition module via by LabVIEW software.Two K-type thermocouples were positioned at the top and bottom of themain chamber to ensure uniform temperature lengthwise thus avoidingnatural convection or buoyancy effect. A 1 inch thick polymer insulationjacket was wrapped around the pre-chamber and main chamber to minimizeheat loss. Fuel (industrial grade hydrogen) and air were introducedseparately to the main chamber using the partial pressure method. Unlikethe main chamber where fuel and air mixed in the chamber itself,fuel/air in the pre-chamber was premixed in a small stainless steelmixing chamber (2.5 cm diameter, 10 cm long) prior going intopre-chamber.

Diaphragm Assessment

After an electric spark ignites the fuel/air mixture in the pre-chamber,pre-chamber pressure starts to rise. Because the volume of thepre-chamber is very small and the initial flow field is quiescent,combustion in it is very likely to occur through the propagation of alaminar flame. Once pre-chamber pressure reaches the rupture pressure ofaluminum diaphragm, the diaphragm bursts and the pressure differencebetween pre and main chamber results in a transient compressible jetwith large density ratio with respect to the relatively cold fuel/airmixture in the main chamber. The jet further penetrates into the mainchamber and could possibly ignite the mixture in the main chamber underfavorable conditions. The jet properties, such as temperature, mean andfluctuating velocities are largely influenced by the pre-chambercombustion process, orifice geometry.

An accurate assessment of diaphragm rupture time is required in order tocalculate precise ignition delay. Ignition delay is defined as the timerequired from the time of diaphragm rupture to the instant of mainchamber ignition. A series of tests were conducted to find out when andat what conditions the thin aluminum diaphragm will rupture. A potentialdifference of 5V was applied using National Instrument DAQ module(NI-9263) to the aluminum diaphragm via two thin copper wires touchingthe diaphragm. As the diaphragm ruptured, copper wires lost contact withaluminum. As a result, voltage dropped sharply marking the event ofrupture. The rupture time is defined as the time interval between thecompletion of the electric spark in the pre-chamber and the rupture ofthe diaphragm. It was found the rupture time for hydrogen/air mixturesto be 2.6±0.1 milliseconds, consistent for all test conditions.

High-Speed Schlieren and OH* Chemiluminescence Imaging

A customized trigger box synchronized with the CDI spark ignition systemsent a master trigger to two high-speed cameras for simultaneousSchlieren and OH* chemiluminescence imaging. The main chamber wasinstalled with four rectangular (5.5″×3.5″×0.75″) quartz windows (typeGE124) on its sides for optical access. High quality UV transparent (85%UV transmission at 240 nm) quartz windows were used. One pair of thewindows was used for ztype Schlieren system. Z-type Schlieren systempositions light source, mirrors, test section and camera in a “Z” shape.Another pair was selected for simultaneous OH* chemiluminescencemeasurement. The high-speed Schlieren technique was used to visualizethe evolution of the hot jet as well as the ignition process in the mainchamber. The system consisted of a 100W (ARC HAS-150 HP) mercury lamplight source with a condensing lens, two concave parabolic mirrors (6″diameter, focal length 1.2 m), and a high-speed digital camera (VisionResearch Phantom v7). Schlieren images were captured with a resolutionof 800×720 pixels with a framing rate up to 12000 fps.

The simultaneous high-speed OH* chemiluminescence measurement provided abetter view of the ignition and flame propagation processes. Ahigh-speed camera (Vision Research Phantom v640) camera, along withvideo-scope gated image intensifier (VS4-1845HS) with 105 mm UV lens,were utilized to detect OH* signals at very narrow band 386 ±10 nmdetection limit. The intensifier was externally synced with the cameravia high-speed relay and acquired images at the same frame rate (up to12,000 fps) with the Phantom camera. A fixed intensifier setting (gain65,000 and gate width 20 microseconds, aperture f8) was used allthrough.

Hot Wire Pyrometry (HWP) and Infrared (IR) Imaging

The Hot Wire Pyrometry (HWP) technique provides a time resolvedtemperature field along a line during jet propagation. Planartime-dependent radiation intensity measurements of the flame wereacquired using an infrared camera (FLIR SC6100) with an InSb detector.The view angle of the camera was aligned perpendicular to the flame axis(50 cm from the burner center to the camera lens) such that the halfview angle of the camera is less than 10 deg. The radiation intensitydetected by each pixel of the camera focal plane array can beapproximated by a parallel line-of-sight because of the small viewangle. The spatial resolution is 0.2×0.2 mm² for each pixel. The bandpass filter was used to measure the radiation intensity of water vaporat the wavelength of 2.58±0.03 μm.

Schlieren Particle Image Velocimetry (SPIV)

In Schlieren PIV (SPIV) method a turbulent flow field containingturbulent eddies serve as PIV particles. These self-seeded successiveSchlieren images with short time delay, Δt can be correlated to findinstantaneous velocity field information. Due to path integrated natureof Schlieren an inverse Abel transformation is required to find truevelocity field. A z-type Herschellian high-speed Schlieren system wasused for Schlieren PIV. The Schlieren system consisted of a 100 Wattmercury arc lamp (Q series, 60064-100MC-Q1, Newport Corporation, Model6281) light source with a condensing lens assembly (Q Series, F/1, FusedSilica, Collimated, 200-2500 nm), two concave parabolic mirrors (6″diameter, aperture f/8, effective focal length 1219.2 mm), a knife-edge,an achromatic lens (f=300 mm) to collimate the light, a beam splitter(1″ cube, Thorlabs PBS251) and two identical high speed CCD cameras(v711, Vision Research Phantom). Utilization of two high speed cameraslie in precise controlling of the inter-frame delay, Δt. A small Δt isessential in order to resolve high exit jet velocity, U₀.

The ultra-lean fuel air ignition results for the different type ofnozzles were obtained and presented in Table 1.

TABLE 1 Nozzle type, dimension, and the measured lean ϕ_(limit) for thedifferent nozzles Nozzle # Type d_(inlet) mm d_(throat) mm d_(exit) mm$\frac{A_{e}}{A_{t}}$ ϕ_(limit)* 1 Straight 1.5 1.5 1.5 NA 0.34 2Straight 3 3 3 NA 0.31 3 Convergent 3 NA 1.5 NA 0.29 4 C-D 3 1.5 3 40.22 5 C-D 3 1.5 4.5 9 0.23 6 C-D 3 1.5 6 16 0.29 7 C-D 3 1.5 3.75 6.250.22 8 C-D 3 1.5 7.5 25 0.32

From the data presented in Table 1, it was surprisingly found that theconverging-diverging type supersonic nozzles 4, 5, and 7 with the arearation range of 4-9 provided the best ignition result that led to thelowest lean fuel-air ratios of 0.22-0.23. Intuitively, with regard toC-D nozzles, since temperature rise increases with an increase in thearea ratio of a nozzle, it was expected that performance would improvewith an increase in the area ratio of the nozzles. Hot Wire Pyrometry(HWP) technique was used to measure the jet temperature at a locationthat is 4 mm downstream of the nozzle exit. The results surprisinglyshowed that nozzle 4 and nozzle 5 (i.e., nozzles with area ratio 4 and9) exhibit higher temperatures at the centerline than the other nozzles.

Radiation intensity (radiation intensity measurement is equivalent totemperature, higher the temperature, higher is radiation intensity)along the jet centerline in axial (i.e., jet propagation) direction wasalso measured for each nozzles. For straight nozzles (nozzle 1 and 2),the radiation intensity drops in a monotonic fashion, indicating thetemperature of the jet keep decreasing as a result of mixing between thehot jet and the cold ambient mixture. However, for nozzles 3, 4 and 5,the measured radiation intensity first fluctuates near the nozzle exitdue to the presence of shocks, for which the static temperatureincreases downstream of the shock. It then increases rapidly at alocation further downstream, indicating the establishment of a highertemperature zone at that location. Resulted ignition of the main chamberlean mixture was observed to take place at this ‘high-temperature zone’for nozzles 3, 4 and 5. In other words, this ‘high-temperature zone’downstream of the nozzle exit is responsible for reducing the lean limitof the main chamber mixture by using a supersonic nozzle.

Radiation intensity along the jet centerline in axial explains whysupersonic jets may ignite a lean mixture. Supersonic jets contain shockstructure. The property of the hot gas changes across a shock. Thestatic temperature rises across the shock. In the presence of a seriesof shock structures, the static temperature keeps on rising, and at theend of final shock the increases static temperature creates a‘high-temperature zone.’ Since velocity near this zone is smallercompared to jet exit, and the mixing is enhanced; the hot supersonic jetgets time to mix with lean fuel/air mixture and has sufficient time forthe chemistry to occur. Thus, the ignition occurs due to a favorablecondition of two properties, 1) high-temperature zone, 2) reducedvelocity/turbulence. C-D nozzles with area ratio between 4 to 9 achieveboth the conditions. Thus, these nozzles can extend the flammabilitylimit. However, the C-D nozzles with higher area ratio create avigorously supersonic jet. Thus, even though they too create ahigh-temperature zone, due to a strong velocity/turbulent field thehigh-temperature zone failed to ignite unburned charge in the mainchamber. Excessive turbulence extinguishes all the ignition kernels.Thus, a moderate turbulence created by C-D nozzles with area ratiobetween 4 to 9 could extend the lean flammability limit of hydrogen-air.

Since ignition delay generally increases with a decrease in theequivalence ratio, it was further expected that extension of the leanflammability limit would result in an increase in the ignition delaytime. However, the investigations indicated that the increase inignition delay time when using supersonic jets relative to subsonic jetswas extremely small, and for practical purposes may be considerednegligible.

Based on observations during the investigations, an ignition Damkohlernumber was determined below which ignition was unlikely to occur andabove which the ignition probability was high. The Damkohler number isused herein as the ratio of the characteristic flow timescale to thecharacteristic chemical reaction timescale, which represents thecompetition between the turbulent mixing and chemical reactions. Forexample, if the temperature of a jet drops too rapidly, it may not becapable of igniting the main chamber mixture. Concurrently, if thevelocity of the jet is too high, the chemical reaction may not haveenough time to occur. The Damkohler number is independent of pre-chamberconfiguration and operating condition, such as chamber pressure andtemperature, orifice size, pre-chamber volume, and geometry. Thisnon-dimensional parameter can be used to promote successful ignition inthe main chamber and therefore is believed to be beneficial for thedesign and optimization of pre-chambers for combustion engines. In theinvestigations leading to the present invention, a minimum Damkohlernumber of eleven was determined to be required to result in main chamberignition.

By incorporating supersonic fuel nozzles into dual chamber engines,improvements to combustion efficiency and reductions to NOx emissionsmay be achieved.

In summary, a vital finding of the present disclosure is the extensionof lean limit, ϕ_(limit) and lower ignition delay of the leanhydrogen/air mixture in the main chamber by using a supersonic nozzlethan a straight nozzle. Ignition in the main chamber was achieved forϕ=0.22 using a supersonic nozzle. Simultaneous Schlieren and OH*Chemiluminescence results show ignition initiates from the side surfaceof the hot jet. Due to the presence of shock structures at the exit ofsupersonic jet, supersonic jet exit temperature is higher than straightnozzles. Increase in the static temperature behind the shocks thusescalates ignition probability, which is the main reason that the leanlimit can be further reduced. Moreover, converging and C-D nozzles witha unique area ratio range of 4-9 created a high temperature zonedownstream of shocks responsible for initiation of ignition. This mayhelp better controlling of the ignition location and ignition delays anddesign a better pre-chamber for lean combustion,

Those skilled in the art will recognize that numerous modifications canbe made to the specific implementations described above. Theimplementations should not be limited to the particular limitationsdescribed. Other implementations may be possible.

1. A supersonic ignition device comprising a supersonic nozzle, whereinthe supersonic nozzle has a converging-diverging geometry and an arearatio between four and nine, and the length of the supersonic nozzle is10-30 mm, and the supersonic nozzle is configured to be capable ofigniting an ultra-lean fuel-air mixture.
 2. The supersonic ignitiondevice of claim 1, wherein the length of the supersonic nozzle is 10-20mm.
 3. The supersonic ignition device of claim 1, wherein the supersonicnozzle has a throat, wherein the throat has a diameter of 1.0-2.0 mm. 4.A supersonic jet combustion engine comprising a pre-chamber, asupersonic nozzle with a length between 10-30 mm, and a main chamber,wherein the supersonic nozzle has a converging-diverging geometry and anarea ratio between four and nine, wherein the supersonic jet combustionengine is configured to generate a jet of combustion products from afirst fuel/air mixture in the pre-chamber, through the supersonicnozzle, and into the main chamber with at least supersonic velocity (atleast Mach 1) to ignite a second fuel/air mixture in the main chamber.5. The supersonic jet combustion engine of claim 4, wherein the secondfuel/air mixture is an ultra-lean fuel mixture with a fuel/airequivalence ratio of equal to or less than 0.4.
 6. The supersonic jetcombustion engine of claim 4, wherein the second fuel/air mixture has afuel/air equivalence ratio of 0.22-0.29.
 7. The supersonic jetcombustion engine of claim 4, wherein the second fuel/air mixture has afuel/air equivalence ratio of 0.22-0.23.
 8. The supersonic jetcombustion engine of claim 4, wherein the supersonic nozzle has a lengthbetween 10-20 mm.
 9. The supersonic jet combustion engine of claim 4,wherein the supersonic nozzle has a throat, wherein the throat has adiameter of 1.0-2.0 mm.
 10. A method of operating a supersonic jetcombustion engine, wherein the method comprises: a) igniting a firstfuel/air mixture in a pre-chamber; b) introducing a jet of combustionproducts from the first fuel/air mixture from the pre-chamber, through asupersonic nozzle with a length between 10-30 mm, and into a mainchamber with at least supersonic velocity (at least Mach 1), wherein thesupersonic nozzle has a converging-diverging geometry and an area ratiobetween four and nine; wherein the jet with supersonic velocity causescombustion of a second fuel/air mixture in the main chamber, wherein thesecond fuel/air mixture is an ultra-lean fuel mixture with a fuel/airequivalence ratio of equal to or less than 0.4.
 11. The method of claim10, wherein the second fuel/air mixture is an ultra-lean fuel mixturewith a fuel/air equivalence ratio of 0.22-0.29.
 12. The method of claim10, wherein the second fuel/air mixture is an ultra-lean fuel mixturewith a fuel/air equivalence ratio of 0.22-0.23.
 13. The method of claim10, wherein the supersonic nozzle has a length between 10-20 mm.
 14. Themethod of claim 10, wherein the fuel is hydrogen, gasoline or naturalgas.
 15. The method of claim 10, wherein the fuel is hydrogen
 16. Themethod of claim 10, wherein the jet results in a Damkohler number ofequal to or greater than
 11. 17. The method of claim 10, wherein thesupersonic nozzle has a throat, wherein the throat has a diameter of1.0-2.0 mm.